1. Field of the Invention
The field of this invention relates to apparatus and processes for producing petroleum coke. More particularly, this invention relates to the recovery of C.sub.1 --C.sub.7 hydrocarbon components present in coker blow down gases.
2. Prior Art
Petroleum coke is produced by heating hydrocarbon residues having boiling points generally above about 750.degree. F. Coker vessels are employed which contain the heated hydrocarbons at temperatures in the range of about 750.degree. F. to about 850.degree. F. Temperatures of at least about 750.degree. F. are required in order to produce any significant amount of coke from petroleum hydrocarbons. The rate of coke formation generally increases with temperature.
During coke formation some of the petroleum residues break down into lower molecular weight hydrocarbons. Some of these lower molecular weight hydrocarbons are recovered and become part of a liquid product produced.
When the coke vessel has filled to a desired amount with coke, a cleaning process to remove the coke deposits is initiated. This includes first discontinuing a flow of residuum hydrocarbon to the coker vessel and then employing a stripping stream, e.g. stream, to remove any hydrocarbons which have not been converted to coke from the vessel. Most of the hydrocarbons stripped by this stream is taken to a fractionator or distillation column. However, at some point the stripping stream and stripped hydrocarbons are transferred to a coker blow down system instead of the fractionator column. The point in time for changing from the fractionator to the coker blow down system is somewhat arbitrary and depends upon the cost of fractionating water and hydrocarbon in the column versus discarding the hydrocarbon which would otherwise be recovered.
Conventionally, the typical blow down recovery system contains a condenser and a knock-out pot, e.g. an accumulator-separator, and conduits to a flare. Noncondensable gases, often referred to as flare gases, which are separated in the knock-out pot, comprise the nonliquefiable portion of the materials taken from the coker vessel during clean up. Conventionally, the non-liquefiable portion so separated is vented to a flare and burned in the air.
The costs for recovering useful components from such flare gases are usually prohibitive. Efforts to find ways of economically recovering useful fuel gas, comprising predominately C.sub.1 -C.sub.4 hydrocarbons, from flare gases has not been successful because of the prohibitive capital costs required by currently known and practiced methods. Also, the large variation in both the quantity and pressure of flare gases over the cycle for producing coke makes it very difficult to recover these gases. Also of some importance is the sulfur content of presently burned flare gases. The greater the sulfur content, the more undesirable environmentally is the burning of such flare gases. Though the pollution contribution from burning high sulfur content flare gases is small, the cumulative effect of it plus other refinery operations can in environmentally sensitive areas require costly retrofitting to lessen sulfur oxide emissions.
It is an object of this invention to provide a cost effective method or process for recovering useful fuel components from flare gases.
It is an object of this invention to lower the sulfur content of flare gases.
Other objects of this invention are clear to one of skill in the art based upon the teachings of this specification.